can't create a branch policy

### **This is autogenerated. Please review and update as needed.**

## Describe the bug

**Command Name**
`az repos policy create
Extension Name: azure-devops. Version: 0.18.0.`

**Errors:**
```
Expecting value: line 1 column 1 (char 0)
Traceback (most recent call last):
pip-install-yh2ypeu1\knack\knack\cli.py, ln 215, in invoke
cli\core\commands\__init__.py, ln 654, in execute
cli\core\commands\__init__.py, ln 718, in _run_jobs_serially
cli\core\commands\__init__.py, ln 709, in _run_job
azext_devops\dev\common\exception_handler.py, ln 31, in azure_devops_exception_handler
reraise(*sys.exc_info())
...
CLI2\lib\json\decoder.py, ln 357, in raw_decode
raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
```

## To Reproduce:
Steps to reproduce the behavior. Note that argument values have been redacted, as they may contain sensitive information.

- _Put any pre-requisite steps here..._
- `az repos policy create --config {} --org {} --project {}`

## Expected Behavior

## Environment Summary
```
Windows-10-10.0.18362-SP0
Python 3.6.6
Installer: MSI

azure-cli 2.9.1

Extensions:
azure-devops 0.18.0

```
## Additional Context

```json
{
"isBlocking": true,
"isDeleted": false,
"isEnabled": true,
"isEnterpriseManaged": false,
"revision": 1,
"settings": {
"buildDefinitionId": 41,
"displayName": null,
"filenamePatterns": [
"/src/AuthenticationServer/*",
"/src/AuthenticationServer.sln",
"/pipelines/AuthServer-onCommit.yml"
],
"manualQueueOnly": false,
"queueOnSourceUpdateOnly": true,
"scope": [
{
"matchKind": "Exact",
"refName": "refs/heads/test",
"repositoryId": "e3d9b748-845e-4867-8085-b065f9fea569"
},
{
"matchKind": "Exact",
"refName": "refs/heads/test2",
"repositoryId": "e3d9b748-845e-4867-8085-b065f9fea569"
}
],
"validDuration": 720.0
},
"type": {
"displayName": "Build",
"id": "0609b952-1397-4640-95ec-e00a01b2c241",
}
}
```